U.S. Patent application Ser. No. 07/439,704 appertains to the imaging of polyester offset printing blanks on laser printers and their preparation and use as offset printing plates. Suitable printing blanks were those such as described in U.K. Application GB 2110161A. (The inventor of the present invention and both of the above U.S. and U.K. applications is M. Figov).
Present computer technology permits and encourages the collection of data from scanners, videos and directly inputted text so that all types of information origination can be used for hard copy composition, assemblage and manipulation within the computer.
In order to generate the hard copy from such inputs, the computer is electronically coupled to a laser printer by which the digitalised information is written as an electrostatic charge onto a photoconductive drum, usually based on Selenium, and the information then transferred by developing the charged latent image with a toner powder and then onto plain paper where it is fused to ensure stability and permanence.
During the past few years, laser printers have been developed that permit larger size copy with greater resolution of the image than hitherto. Thus, whereas laser printers producing A4 copy were the most common ones available in the 1980's, A3 and larger sizes are rapidly becoming popular in the market. The original A4 laser printers had a resolution related to their capability to write at 300 dots per inch. Laser engines are now available with 600 dots per inch and more recently 1200 dots per inch engines have been developed. These developments have further promoted interest in the production of printing plates by this means.
A polyester printing plate is described in U.S. application Ser. No. 07/439,704 based on U.K. Patent Application publication no. GB-2110161A. This plate can be fed directly into the laser printer, where it receives information for printing by the offset litho process. The plate comprises a polyester base on which a mixture of zinc oxide and binder is coated. After laser printer imaging, the plate is treated with a conversion fluid which renders the background hydrophilic whilst the toner image remains oleophilic. This conversion technology is based on the well-known electrophotographic offset paper plate printing process as described for instance in U.S. Pat. Nos. 3,522,062 and 4,579,591.
Although there are other direct-to-plate systems of collecting data directly from a computer and depositing it onto a printing plate (for instance as described in U.S. Pat. Nos. 4,149,798 and 4,774,532) they require dedicated laser imagers that have to be manufactured and sold specifically for their application. The inventor's (M. Figov) previously described inventions utilize an existing, growing and developing laser engine population that finds wider application as a disseminator of information via a multifarious selection of communication media which can now combine "one off" or low number multiple copying with high volume printing. Moreover, the movement to larger format size and higher resolution favours the printing process, bearing in mind that the size of a printing plate must exceed that of the required copy size in order to permit gripping the plate at its head and tail, and in order to avoid the edge of the plate printing an unwanted line on the paper. Thus, A4 laser printers can only produce printing plates of a smaller print size than A4 and have to be specially adjusted to give sufficient plate length to enable attachment of both ends of the plate on the offset litho cylinder.
U.S. Patent application Ser. No. 07/439,704 specifically tackles a particular problem experienced in the system-- namely the appearance of small background dots of toner powder that are fused into the background. These dots become a problem during the printing process in that they reproduce on the prints as unwanted image background. An emulsion was patented which when applied to the etched plate reduces the size of these background dots to minimize or eliminate their visibility on the final print as well as sharpening the print and increasing the oleophilic properties of the toner image.
According to the present invention it is possible to design the formulation of the printing plate so as to considerably reduce the background dot formation to such an extent that the emulsion according to U.S. application Ser. No. 07/439,704 needs not necessarily be applied.
In addition, the present invention relates to formulations that have the following advantages over those previous described. They are:
(i) Ease of image erasure after the printing plate has been used This enables correction to be done, and even large extraneous dots to be eliminated. PA0 (ii) Ease of automatic feeding of plates through the laser printer. Polyester plates stick together through static charging and the formulation according to the present invention helps eliminate this. PA0 (iii) Compatibility with alcohol based founts. PA0 (iv) Improved u.v. transparency so that the plate can also be used as an offset intermediate film for making very long run aluminum plates. PA0 (1) An electroconductive base material. This is usually (commercially) a high wet-strength paper impregnated with an electroconductive resin. According to patent literature (e.g. U.S. Pat. No. 4,457,992) aluminum metal as a base has also been used. An example of a commercially available paper base is Electrostatic Plate Base D7481 from Intermills International. PA0 (2) Zinc oxide of an electrophotographic grade, capable of being dye sensitized to respond to various wavelengths of light. Such a zinc oxide is Photox 801 from the New Jersey Zinc Company-- see U.S. Pat. No. 3,345,162. PA0 (3) A suitable resin binder of good dielectric properties. Commercial examples are given further on. PA0 (4) A dye or combination of dye sensitizers (for instance rose bengal, bromophenol blue and fluorescein) which regulate the spectral sensitivity of the coating making it sufficiently electrophotographic for satisfactory performance. PA0 (1) A polyester base. This is by nature electrically non-conductive. PA0 (2) Zinc oxide. This is not necessarily an electrophotographic grade, but its essential feature is that it can be converted by an electrostatic conversion etch to give a water insoluble but water receptive layer. PA0 (3) A suitable resin binder. This need not necessarily have good dielectric properties but must have good adhesion to polyester, provide good bonding to the laser toner powder, and have mechanical strength to remain intact during the offset printing process.
The polyester laser printing plate referred to in the above mentioned U.S. application Ser. No. 07/439,704 was marketed under the name of Plazer (hereinafter called "Plazer"). The basis of the Plazer plate as far as composition is concerned can be considered as comprising three essential elements. The first is the polyester base. This material is flexible and sufficiently thin (80 to 150 microns) to be fed into the laser printer but not so thin as to be damaged by the heat fusing rollers through which it must pass. Yet it must also be sufficiently robust to withstand successive impacting during the printing process.
The second and third elements of the Plazer are the zinc oxide and the binder resin which combine together to give the coating that must exhibit good reception and adhesion to the toner image as it is formed in the laser printer, must give good conversion using an electrostatic conversion etch to give a clean background on printing, and must withstand the mechanical and chemical forces applied to the surface of the plate during printing on the offset printing machine.
Thus, the combination of the three elements results in a printing blank that receives good quality images from laser printers and then can be chemically treated with etch to give a clear running printing plate which is easy to use and which is robust and durable for more than 15,000 copies.
It is recognized that anyone skilled in the art reading GB Application No. 2110161A and U.S. application Ser. No. 07/439,704 would recognize similarities between the zinc oxide / resin layers therein described which have properties of conversion to hydrophilic layers with oleophilic images and those of commercially available electrophotographic paper offset litho plates which utilize these same principles (as depicted, for instance in the aforementioned U.S. Pat. No. 3,522,062).
However, a close examination of the technology of the electrophotographic offset paper plate reveals that such plates comprise a number of essential features that are not features of those plates used in the laser printing process described in the inventor's previous invention. Thus, electrophotographic printing plates need the following essential features:
When all these features are combined, the electrophotographic printing blank is such that it will accept a charge of about 400 volts and that this charge will be held with little loss (dark decay) for a period of some seconds U.S. Pat. No. 3,615,419 reports satisfactory results for a 350 volt charge acceptance and a dark decay rate of 4 volts per second. On exposure to light (during the imaging process) the material becomes conductive so that non image or background areas rapidly lose their charge. This is shown diagrammatically in U.S. Pat. No. 4,457,992. The image is subsequently developed with a toner powder which is thermally fused onto the plate.
As described above, in the type of plate involved in the laser printing process of U.S. application Ser. No. 07/439,704, the essential features are somewhat different.
They are as follows:
When these elements are combined, the electrical properties are entirely different from those of the electrophotographic materials previously described. Primarily, these are due to the insulative nature of the polyester base. It has been found, for instance, that if the coating formulations described in GB 2110161A or in U.S. Pat. No. 3,540,886 are applied onto polyester, the coating will hold a charge of up to 1500 volts. These layers then have a dark decay over a minute of less than 5% and a light decay of the same order. If the same above mentioned coatings are then coated onto an electroconductive paper, (Electrostatic Plate Base D7481 -- Intermills), whether they are dye sensitized or not they exhibit an initial charging of around 400-500 volts. A possible explanation for this difference is that the polyester is impermeable and the coating thus resides entirely on its surface, whereas it penetrates into the conductive paper so that its intrinsic electrical properties are modified by those of the paper.
It was also discovered that there was a significant difference in surface resistivity between electrophotographic offset plates and the polyester "Plazer" printing plate. The former was measured at 10.sup.10 ohms per square and the latter at 10.sup.12 ohms per square. In the Patent Application GB 2110161A which describes an offset plate for use with a plain paper copier that used mono-component toner, the essential electrical property was volume resistivity that had to be greater than 10.sup.12 ohms. cms. Surface electrical properties are not recorded. However, the offset litho plates of 2110161A were specifically designed to fit into the special development system of cold pressure fusing of mono component toner because the carrier in mono component toners is deposited with the toner onto the print and thus has entirely different electrical characteristics to the toner of the 2 component system. The electrical requirements of the receptor sheets be they paper or plate, are different in the two cases. Present laser printer technology is not based on this system.